Rats will not bite any more than mice. In fact, people who've had ANY issues with feeding live tend to have more problems with mice than rats.

The only time you have to worry about anything biting or injuring your snake is if you don't keep an eye on the feeding or leave it in there for too long.

Feeding a small/medium rat would be sufficient. Feeding multiple food items can be difficult on their system and it's generally cheaper and easier to feed one item per round.
Use the rule of thumb; feed prey thats about the same thickness as the thickest point on the snakes body. A little more or less isn't going to do any harm, but going too high over or below can be dangerous.

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